1. Field of the Invention
This invention relates to radio receivers, and more particularly, to automatic frequency compensation in radio receivers.
2. Description of the Related Art
Automatic frequency compensation (AFC; sometimes referred to as automatic frequency control) is a technique used in radio communications system to minimize the difference between the frequency of an oscillator and a desired frequency. In radio receivers, AFC may be used to control the frequency of a local oscillator such that it is as close as possible to the desired radio frequency such that the received signal frequency is in the center of a frequency response of a channel filter. In one embodiment of a basic AFC loop, a local oscillator may be implemented using a voltage-controlled oscillator (VCO). A difference between the receiver center frequency and the incoming signal frequency may be sensed and converted into a control voltage. This control voltage may then be provided to the local oscillator in order to adjust the frequency. In general, the AFC loop may operate as a closed-loop feedback system.
In digital radio communications systems, a known data sequence referred to as a preamble may be transmitted to the receiver. The preamble may be periodically transmitted (e.g., with each frame of data). The preamble may be used for synchronization of the incoming data. In addition, some digital communications systems may be utilized to settle the AFC loop. Incoming data may be sampled, for example, every two symbols, with a difference computed between the samples in order to determine an offset, which can then be used to set the control voltage provided to the VCO.